To be completely honest, a lot of people including Fi & I are in the same boat. We don't need anything; in fact, we already have two of everything from when we moved in together two years ago. I can understand where you are coming from in not wanting gifts because there is nothing you "need"... but in some guests minds they feel the need to give you a gift whether it be monetary or otherwise.
I agree with Amarillis that I just can't fathom asking for money. In our case, we just said "Your presence is present enough!". That's all that was needed to be said; if a guest chooses to buy us gifts or give us money then that is up to them AND we will appreciate whatever they give us or do not give us.
As Kristy! said, there are tons of threads on this ... I highly suggest doing a search to see what other brides are doing; there must be a more creative solution than asking for money that will work for you.
Good Luck!!